1. Field of the Invention
The present invention relates to a protective and/or diagnostic device for a multi-voltage electrical system of a motor vehicle, a corresponding multi-voltage vehicle electrical system, and a method for operating a multi-voltage vehicle electrical system.
2. Description of the Related Art
Multi-voltage electrical systems for motor vehicles are known in general. They are used because a single voltage supply is often no longer sufficient due to the increasing number of electrical consumers in motor vehicles and/or consumers having different nominal voltages are used. Multi-voltage vehicle electrical systems typically include subnetworks having different voltage levels of 12 V and 42 V, for example. It is also known to connect subnetworks via one DC/DC converter, so that they may be supplied via a shared generator.
Within the scope of this application, dual-voltage vehicle electrical systems, i.e., multi-voltage vehicle electrical systems having only two subnetworks of different voltage levels, are predominantly considered. The present invention is, however, not limited to dual-voltage vehicle electrical systems, as elucidated below.
The subnetworks are frequently connected to one shared ground conductor. In practice, this is not easily preventable, since electrical machines, for example, which have their ground connection via the housing and a mechanical connection via the internal combustion engine, would otherwise have to be insulated electrically against the internal combustion engine. An electrical insulation of this type in a place which is subjected to great mechanical stresses is, however, associated with considerable complexity, since the material properties of an insulating material are not always reconcilable with the mechanical requirements.
However, a problem arises due to the shared ground connection if the shared ground wire is disrupted. In this case, a polarity reversal of the electrical consumers may occur in a subnetwork having a lower voltage level, as elucidated below in conjunction with FIG. 1.
To protect against such a polarity reversal, published German patent application document DE 199 61 622 A1 proposes in the case of a dual-voltage vehicle electrical system, which has two electrically connected subnetworks of different voltage levels in which electrical consumers are situated, to place a protective diode into the vehicle electrical system having the lower voltage level in parallel to the electrical consumers.
In this way, the polarity reversal voltage of the electrical consumers is limited to the conducting-state voltage of the diode in the event of a grounding fault. A fuse which is coordinated with the maximum current intensity of the diode may be situated in the vehicle electrical system having the lower voltage level. A similar approach using a protective diode is disclosed in published German patent application document DE 101 38 477 A1.
Published international patent application document WO 01/47084 A1 proposes the use of a voltage measuring device in order to recognize an overvoltage in a subnetwork having the lower voltage level. If this voltage measuring device recognizes an overvoltage, the consumers relevant in this case are disconnected from the network. Published European patent application document EP 1 453 171 A1 proposes corresponding switching means for the consumers' protection.
However, the measures known from the related art frequently prove to be complex and ineffective. Therefore, the need continuously exists to provide possibilities of reliably recognizing and/or preventing a polarity reversal of consumers in the case explained above.